Molded plastic closures have found increasingly widespread acceptance in the marketplace for all manner of applications, including widespread use on containers having carbonated or otherwise pressurized contents. This type of application can be somewhat problematic, in that gas pressure within the container can tend to deform a plastic closure during normal storage, shipment, and retail sale. This type of deformation of the closure, which is in the nature of a cold flow phenomenon as the gas pressure acts against the inside surface of the closure, can result in outward bowing or "doming" of the top wall portion of a closure. This deformation can inhibit the sealing integrity of the closure, since the deformation can result in diminished sealing engagement between the inside surface of the closure top wall and the upper surface of the associated container.
U.S. Pat. No. 4,497,765, hereby incorporated by reference, discloses a so-called composite closure, and method of formation, which has proven to be very highly effective for packaging of carbonated beverages and the like. This type of closure is particularly configured to form a "top/side seal" with an associated container, by the provision of a sealing liner positioned adjacent the inside surface of the top wall of an outer closure cap of the composite closure. The sealing liner includes an annular sealing portion at the periphery thereof which presents a generally inwardly facing sealing surface. The liner is thus configured to make sealing engagement with both upwardly facing and outwardly facing surfaces of the associated container, for the formation of the desired top/side seal. Experience has shown that in the event that this type of closure is subjected to doming or like deformation, the sealing integrity of the closure is maintained by the inwardly facing sealing surface of the closure liner, even if deformation of the closure results in diminished sealing engagement between the liner and the upwardly facing surface of the container.
While the above-described composite closure construction has proven to be very commercially successful, it is desirable to provide these types of closures with the best possible sealing performance that can be achieved at acceptable cost. Experience has shown that under some storage and handling conditions of containers, closures can be subjected to top-loading which may disrupt the sealing integrity of closures forming top/side seals, as described above. The present invention is directed to a composite closure configured to provide enhanced sealing under the various types of conditions which are typically encountered during shipment and storage of containers having carbonated contents on which the closures are used.